1. Field of the Invention
The present invention relates to an image forming apparatus which includes a developing unit adapted for removal mounting of plural developing devices each storing a toner of the same color, and which performs a printing operation using the toner in the developing device mounted in the developing unit, as well as to an image forming method thereof.
2. Description of the Related Art
Conventionally, image forming apparatuses for forming images using a plurality of developer devices have been widely known in the art. For instance, Japanese Patent Application Laid-Open Gazette No. 2003-215862 discloses a color image forming apparatus equipped with a rotary developing unit having four developer devices radially arranged about a rotary shaft thereof This apparatus operates as follows. The rotary shaft of the developing unit is driven into rotation for selectively positioning one of the four developer devices at an opposed position to a latent image carrier such as a photosensitive member, so as to develop a latent image on the latent image carrier. The resultant image is transferred onto an intermediate transfer medium. The same developing and transferring processes as the above are repeated in cycles while switching from one developer device to another, whereby toner images of plural colors are superimposed on each other to form a color image.
In the image forming apparatuses of this type, a proposal has been made to mount a plurality of developing devices each storing a toner of the same color so as to use the apparatus as an image forming apparatus exclusive to monochromatic image formation. Japanese Patent Application Laid-Open Gazette No. 2002-351190, for example, discloses an image forming apparatus wherein plural developing devices each storing a toner of the same color are mounted in the developing unit. The apparatus forms monochromatic images using any one of the developing devices while switching from one developing device to another as needed.
In order to print an image with excellent quality using the toner in the developing device, a print preparatory process for bringing the developing device into a state usable for printing operation must be conducted prior to the printing operation. In the image forming apparatuses of this type, there is known a phenomenon, for example, that when an image forming operation is performed using a developing device left standstill for long, image density variations are encountered in an initial stage of the operation. Hence, it is necessary to carry out a process for eliminating this phenomenon whenever needed. For achieving a high image quality, the conditions of printing operation of the apparatus need be adjusted according to the characteristics of the developing device used.
In this connection, a variety of techniques for conducting favorable print preparatory processes have heretofore been proposed with respect to an image forming apparatus including one developing device and a color image forming apparatus including a plurality of developing devices individually storing toners of mutually different colors.